Non-contact methods are effective in locating structural defects in metal structures at an early stage in the life of a flaw and allow the appropriate corrective action, such as removing and replacing a defective component, to be accomplished before a defect causes a catastrophic failure. Consequently, where nondestructive testing can be implemented during their operational life, structural components need not be precautionarily oversized and weighted. Nondestructive testing techniques can thus be utilized to maintain a desired level of reliability in a structure while concurrently reducing construction and material costs.
One of the many types of non-contact testing is ultrasonics, in which the interaction between acoustic wave energy and the internal structure of an object is analyzed to predict the physical integrity of the object. Non-contact ultrasonic techniques, such as electromagnetic transducers, are ideal for structural testing at high speeds, at elevated temperatures, and in remote and inaccessible locations. One of the most effective non-contact transducers is the electromagnetic acoustic transducer (EMAT).
An EMAT consists of a conductor which is positioned within a static "biasing" magnetic field (B) near the surface of a conducting material. When an alternating current (I.sub..omega.) is applied to the conductor, eddy currents (J.sub..omega.) are induced within the surface layer of the conducting material. These induced eddy currents, in the presence of a biasing magnetic field, results in a Lorentz force which deflects the moving electrons in a direction defined by the vector product of J.sub..omega. .times.B. The electrons then collide with the ions in the lattice structure of the conducting material, ultimately generating acoustic energy in the form of an ultrasonic wave that propagates through the metal structure. The velocity (v) of the ultrasonic wave is determined by the scalar product of its wavelength (.lambda.) and its frequency (f), i.e., v=.lambda..multidot.f. The frequency of the ultrasonic wave is determined by that of the applied alternating current. Additionally, the orientation of both the biasing magnetic filed and the induced eddy current determine the direction and mode characteristics of the propagating energy.
EMATs have been fabricated with a variety of coil and magnet configurations to suit the requirements of particular applications. U. S. Pat. Nos. 3,850,028, 4,048,847, 4,080,836, 4,092,868, 4,104,922, 4,127,035, 4,184,374, 4,218,924, 4,232,557, 4,248,092, 4,344,663 and 4,593,567, for example, the teachings of which are incorporated herein by reference, illustrate some of the approaches which have been utilized. While EMATs have thus been employed to great advantage in many testing situations, some significant limitations of previous EMAT designs have been identified.
The periodic permanent magnet EMAT, for example, which is best described in U.S. Pat. No. 4,127,035, can be used to generate certain types of ultrasonic waves which are difficult or impossible to produce with other transducer designs. However, the fabrication of a periodic permanent magnet EMAT requires extensive precision machine work to produce a permanent magnet of the proper dimensions for the EMAT. Additionally, an elaborate assembly procedure is necessary because such permanent magnets are comprised of a compilation of smaller magnets, each separated from the rest by a thin layer of non-magnetic insulating material to prevent the generation of eddy currents within the permanent magnet.
Other prior art EMATs have been developed to overcome the fabrication limitations of permanent magnet EMATs, however, they too are not without their own drawbacks. For example, U.S. Pat. No. 4,593,567 provides an EMAT for the touchless testing of metal workpieces by wavelength spectroscopy comprising at least one transducer having a plurality of mutually parallel conductor tracks formed on printed circuit board in which the frequencies and wavelengths for each segment can be preset in a matrix logic circuit. The conductor tracks are produced by multiple windings that are equidistantly spaced for precisely setting the individual wavelengths. The complexity of the logic circuit makes this prior art EMAT very expensive to manufacture and maintain.
A factor to consider in evaluating the efficiency of a coil-type EMAT, is the ability to efficiently convert electrical to acoustic energy and back again (i.e., conversion efficiency or coupling). The more energy retained after the electromagnetic energy is converted to ultrasonic wave form, the more efficient the EMAT, i.e., the less tile system produces unused power. Therefore, it would be advantageous to design an EMAT that has a high conversion efficiency.
Accordingly, it is a general objective of this invention to provide a new and improved electromagnetic acoustic transducer.
Another object of the present invention is to provide an electromagnetic acoustic transducer having coils that are more durable than those found in the prior art.
Another object of the present invention is to provide a method for fabricating an electromagnetic acoustic transducer that is less expensive than prior art methods.
Still another object of the present invention is to provide an electromagnetic acoustic transducer that has a conversion efficiency that is greater than that found in the prior art.